Option Explicit and Option Strict

Keeping Option Explicit turned off can lead to two very bad problems. First, it silently hides typographical errors. If you look closely at the previous code, you’ll see that the statement inside the For loop increments the misspelled variable num_managrs instead of the correctly spelled variable num_managers . Because Option Explicit is off, Visual Basic assumes that you want to use a new variable, so it creates num_managrs . After the loop finishes, the program displays the value of num_managers , which is zero because it was never incremented.

The second problem that occurs when Option Explicit is off is that Visual Basic doesn’t really know what you will want to do with the variables it creates for you. It doesn’t know whether you will use a variable as an Integer, Double, String, or PictureBox. Even after you assign a value to the variable (say, an Integer), Visual Basic doesn’t know whether you will always use the variable as an Integer or whether you might later want to save a String in it.

To keep its options open , Visual Basic creates undeclared variables as generic objects. Then it can fill the variable with just about anything. Unfortunately, this can make the code much less efficient than it needs to be. For example, programs are much better at manipulating integers than they are at manipulating objects. If you are going to use a variable as an integer, creating it as an object makes the program run much slower.

The following code executes two For loops . In the first loop, it explicitly declares its looping variable to be of type Integer. In the second loop, the code doesn’t declare its looping variable, so Visual Basic automatically makes it an object when it is needed. In one test, the second loop took more than 60 times as long as the first loop.

Const TRIALS As Integer = 10000000


Dim start_time As DateTime


Dim stop_time As DateTime


Dim elapsed_time As TimeSpan


Dim i As Integer


start_time = Now


For i = 1 To TRIALS


Next i


stop_time = Now


elapsed_time = stop_time.Subtract(start_time)


MsgBox(elapsed_time.TotalSeconds.ToString("0.000000"))


start_time = Now


For j = 1 To TRIALS


Next j


stop_time = Now


elapsed_time = stop_time.Subtract(start_time)


MsgBox(elapsed_time.TotalSeconds.ToString("0.000000"))

The second compiler directive that influences variable declaration is Option Strict . When Option Strict is turned off, Visual Basic silently converts values from one data type to another, even if the types are not very compatible. For example, Visual Basic will allow the following code to try to copy the string s into the integer i . If the value in the string happens to be a number (as in the first case), this works. If the string is not a number (as in the second case), this throws an error at runtime.

Dim i As Integer Dim s As String s = "10" i = s ' This works. s = "Hello" i = s ' This Fails.